This invention relates to a joint structure of two plate members hemmed in such a manner that one plate member is insetted between a body part and a return part of the other plate member, such as an outer plate part for a vehicle, for example, a bonnet, a trunk lid, etc.
An outer plate member for a vehicle, such as a bonnet, trunk lid, etc. is composed of an outer panel and an inner panel, assembled. The outer panel has a return part at its end portion and a body part. When assembling the outer panel and inner panel, conventionally, the inner panel is insetted at its end portion between the body part and the return part of the outer panel, and then the return part, the end portion of the inner panel and the body part are spot-welded.
In this case, however, where the hemmed outer and inner panels are spot welded, the body part and the return part which are insetting the end portion of the inner panel are nipped by a head electrode and a back electrode for spot welding to conduct an electric current therebetween. This results in a strain owing to application of a pressure by both electrodes and scabs upon spot welding at the spot-welded point. This requires to remove the strain and the scabs after spot welding, which leads to increase in time required for assembling work.
In a production line with less quantity and various kinds of products, a type of the plate members to be conveyed on an assembly line is frequently altered, which means that a point to be welded must be changed. Therefore, the position of the back electrode for supporting the plate members is required to be changed. However, since the back electrode is fixed to a rack for one type of the plate member, an exclusive rack having a differentiated position of the back electrode is prepared for each type of the plate members. Thus, the cost of equipment increases and an efficiency of assembling work lowers owing to a troublesome changing operation of the racks.
It has been devised that after the first plate member is insetted at its end portion between the body part and the return part of the second plate member, the return part of the second plate member and the end portion of the first plate member is laser-welded by irradiating a laser beam at the return part thereof.
In this case, though the problems that the scabs remain on the body part of the second plate member and the position of the back electrode for supporting the plate members must be changed are solved, a thermal strain shall remain at the body part of the second plate member owing to heat of the laser welding.
Further, in a case where the first plate member is adhered to second plate member with an adhesive beforehand, the adhesive generates gas at the time of laser welding. Since the gas diffuses between the first plate member and the second plate member, the adhered plate members shall be separated each other.